Thermally-assisted spin transfer torque memory with improved bit error rate performance

ABSTRACT

Embodiments of the invention are directed to a magnetic tunnel junction (MTJ) storage element having a reference layer formed from a reference layer material having a fixed magnetization direction, along with a free layer formed from a free layer material having a switchable magnetization direction. The MTJ is configured to receive a write pulse having a write-pulse and spin-transfer-torque (WP-STT) start time, a WP-STT start segment duration and a write pulse duration. The WP-STT start segment duration is less than the write pulse duration. The fixed magnetization direction is configured to form an angle between the fixed magnetization direction and the switchable magnetization direction. The angle is sufficient to generate spin torque electrons in the reference layer material at the WP-STT start time. The spin torque electrons generated in the reference layer material is sufficient to initiate switching of the switchable magnetization direction at the WP-STT start time.

DOMESTIC PRIORITY

This application is a continuation of U.S. application Ser. No.15/464,752, filed Mar. 21, 2017, the contents of which are incorporatedby reference herein in its entirety.

BACKGROUND

The present invention relates generally to electronic memory, and morespecifically to spin transfer torque storage elements that provideimproved bit error rate performance.

Electronic memory can be classified as volatile or non-volatile.Volatile memory retains its stored data only when power is supplied tothe memory, but non-volatile memory retains its stored data withoutconstant power. Volatile random access memory (RAM) provides fastread/write speeds and easy re-write capability. However, when systempower is switched off, any information not copied from volatile RAM to ahard drive is lost. Although non-volatile memory does not requireconstant power to retain its stored data, it in general has lowerread/write speeds and a relatively limited lifetime in comparison tovolatile memory.

Magnetoresistive random access memory (MRAM) is a non-volatile memorythat combines a magnetic device with standard silicon-basedmicroelectronics to achieve the combined attributes of non-volatility,high-speed read/write operations, high read/write endurance and dataretention. The term “magnetoresistance” describes the effect whereby achange to certain magnetic states of the MTJ storage element (or “bit”)results in a change to the MTJ resistance, hence the name“Magnetoresistive” RAM. Data is stored in MRAM as magnetic states orcharacteristics (e.g., magnetization direction, magnetic polarity,magnetic moment, etc.) instead of electric charges. In a typicalconfiguration, each MRAM cell includes a transistor, a magnetic tunneljunction (MTJ) device for data storage, a bit line and a word line. Ingeneral, the MTJ's electrical resistance will be high or low based onthe relative magnetic states of certain MTJ layers. Data is written tothe MTJ by applying certain magnetic fields or charge currents to switchthe magnetic states of certain MTJ layers. Data is read by detecting theresistance of the MTJ. Using a magnetic state/characteristic for storagehas two main benefits. First, unlike electric charge, magnetic statedoes not leak away with time, so the stored data remains even whensystem power is turned off. Second, switching magnetic states has noknown wear-out mechanism.

STT is a phenomenon that can be leveraged in MTJ-based storage elementsto assist in switching the storage element from one storage state (e.g.,“0” or “1”) to another storage state (e.g., “1” or “0”). For example,STT-MRAM 100 shown in FIG. 1 uses electrons that have beenspin-polarized to switch the magnetic state (i.e., the magnetizationdirection 110) of a free layer 108 of MTJ 102. The MTJ 102 is configuredto include a reference/fixed magnetic layer 104, a thin dielectrictunnel barrier 106 and a free magnetic layer 108. The MTJ 102 has a lowresistance when the magnetization direction 110 of its free layer 108 isparallel to the magnetization direction 112 of its fixed layer 104.Conversely, the MTJ 102 has a high resistance when its free layer 108has a magnetization direction 110 that is oriented anti-parallel to themagnetization direction 112 of its fixed layer 104. STT-MRAM 100includes the multi-layered MTJ 102 in series with the FET 120, which isgated by a word line (WL) 124. The BL 126 is coupled to the MTJ 102, andthe SL 128 is coupled to the FET 120. The MTJ 102 (which is one ofmultiple MTJ storage elements along the BL 126) is selected by turningon its WL 124.

The MTJ 102 can be read by activating its associated word linetransistor (e.g., field effect transistor (FET) 120), which switchescurrent from a bit line (BL) 126 through the MTJ 102. The MTJ resistancecan be determined from the sensed current, which is itself based on thepolarity of the magnetization direction 110 of the free layer 108.Conventionally, if the magnetization direction 112 of the fixed layer104 and the magnetization direction 110 of the free layer 108 have thesame polarities, the resistance is low and a “0” is read. If themagnetization direction 112 of the fixed layer 104 and the magnetizationdirection 110 of the free layer 108 have opposite polarities, theresistance is higher and a “1” is read.

When a voltage (e.g., 500 mV) is forced across the MTJ 102 from the BL126 to the SL 128, current flows through the selected cell's MTJ 102 towrite it into a particular state, which is determined by the polarity ofthe applied voltage (BL high vs. SL high). During the write operation,spin-polarized electrons generated in the reference layer 104 tunnelthrough the tunnel layer 106 and exert a torque on the free layer 108,which can switch the magnetization direction 110 of the free layer 108.Thus, the amount of current required to write to a STT-MRAM MTJ dependson how efficiently spin polarization is generated in the MTJ.Additionally, STT-MRAM designs that keep write currents small (e.g.,I_(c)<25 micro-ampere) are important to improving STT-MRAM scalability.This is because a larger switching current would require a largertransistor (e.g., FET 120), which would inhibit the ability to scale upSTT-MRAM density.

STT-MRAM technologies have been proposed to reduce the switching currentby improving or increasing the generation of spin torque electrons. Forexample, it is more difficult to change the magnetization direction ofthe MTJ free layer at a normal operating temperature. Accordingly,so-called “thermally-assisted” or “thermoelectric” STT-MRAM has beendeveloped that uses the application of heat to reduce the requiredswitching current. In a known configuration, the thermally-assistedSTT-MRAM includes a MTJ and a tunnel junction programming circuit. TheMTJ includes a reference layer having a fixed magnetization direction, atunnel barrier layer, and a free layer on an opposite side of the tunnelbarrier layer from the reference layer. The free layer includes a firstlayer having a first Curie temperature and a second layer having asecond Curie temperature different from the first Curie temperature. Amaterial's Cure temperature is the temperatures at which the materialbecomes nonmagnetic. The tunnel junction programming circuit isconfigured to apply a current through the MTJ to generate a writetemperature in the MTJ and assist in writing (i.e., switching) themagnetization direction of the MTJ free layer.

Another consideration in STT-MRAM is the bit error rate (BER). Indigital transmissions, the number of bit errors is the number of datastream bits received over a communication channel that have been altereddue to noise, interference, distortion or bit synchronization errors.The BER is defined generally as the number of bit errors per unit time.The basic mechanisms that contribute to the BER of a given STT-MRAMdesign include thermal disturbance, read disturbance, and probabilisticwrite failure. The write process of a spin torque MTJ storage element isinherently stochastic due to thermal fluctuations, which give rise to adistribution of the magnetization of the free layer before and duringswitching. As a result, the time taken by the spin torque MTJ to switchcan have a wide distribution. Therefore, there will be a non-zeroprobability that when a finite duration write pulse is turned off thespin torque MTJ will not have been written, which results in a so-calledwrite error. The probability that a write error takes place for a givenapplied current pulse of a given length is known as the write error rate(WER). It has been estimated that correct operation of an STT-MRAM arraycan require that the WER is less than 10⁻⁹ if there is an errorcorrection circuit (ECC) in the chip. This means that if 10⁻⁹ writepulses are applied, the MTJ reliably writes all but one (1). If there isno ECC, it has been estimated that the WER needs to be less than 10⁻¹⁹.

Returning now to the concept of STT, the spin torque that is applied tothe MTJ free layer is proportional to the sine of θ, where θ is theangle between the free layer magnetization direction and the referencelayer magnetization direction. In a zero temperature environment, whenthe STT-MRAM switching process starts, there is no spin torque because θis equal to zero (0) degrees (i.e., the magnetization directions of thefixed and free layers are parallel) or 180 degrees (i.e., themagnetization directions of the fixed and free layers areanti-parallel), and the sine of zero (0) degrees or 180 degrees is zero(0). In practice, however, there typically exists a very small butfinite temperature that fluctuates by very small amounts. The very smallfluctuations in temperature oscillate the free layer back and forth by avery small amount. Thus, in practice, the sine θ term at the start ofthe write process is not precisely zero (0) but is still very small andnot sufficient to generate spin torque electrons in the fixed layer.However, in a thermally-assisted STT-MRAM, as the write current raisesthe MTJ temperature the free layer magnetization direction begins tochange, and the sine θ term ramps up to become increasingly larger.After a period of time (e.g., about 1 nanosecond (ns)), the 0 term issufficiently large to generate sufficient spin torque to begin theprocess of switching the free layer magnetization direction. This spintorque ramp up period, which is inherent to known STT-MRAMconfigurations results in inherent switching delays that can increasethe WER. Accordingly, a contributing factor to WER in thermally-assistedMTJ storage elements is that no or insufficient spin torque is generatedat the start of the write operation because sine θ, at the start of awrite operation and for a period of time thereafter, is either zero (0)or very close to zero (0).

SUMMARY

Embodiments of the invention are directed to a magnetic tunnel junction(MTJ) storage element. In a non-limiting example, the MTJ includes areference layer formed from a reference layer material having a fixedmagnetization direction, along with a free layer formed from a freelayer material having a switchable magnetization direction. The MTJ isconfigured to receive a write pulse having a write-pulse andspin-transfer-torque (WP-STT) start time, a WP-STT start segmentduration and a write pulse duration. The WP-STT start segment durationis less than the write pulse duration. The fixed magnetization directionis configured to form an angle between the fixed magnetization directionand the switchable magnetization direction. The angle is sufficient togenerate spin torque electrons in the reference layer material at theWP-STT start time. Advantages of the above-described embodiments of theinvention include, but are not limited to, improved WER performance.More specifically, because the above-described embodiments of thepresent invention provide spin torque electrons from the start of thewrite pulse, the likelihood of a write error is less than that in spintorque MTJ configurations that do not provide spin torque electrons fromthe start of the write pulse. Accordingly, spin torque MTJ storageelements according to the above-described embodiments of the inventionprovide improved WER performance.

Embodiments of the invention are directed to a MTJ storage element. In anon-limiting example, the MTJ includes a first reference layer formedfrom a first reference layer material having a first fixed magnetizationdirection, along with a free layer formed from a free layer materialhaving a switchable magnetization direction. The MTJ further includes asecond reference layer formed from a second reference layer materialhaving a second fixed magnetization direction. The MTJ is configured toreceive a write pulse having a WP-STT start time, a WP-STT start segmentduration and a write pulse duration. The WP-STT start segment durationis less than the write pulse duration. The first fixed magnetizationdirection is configured to form a first angle between the first fixedmagnetization direction and the switchable magnetization direction. Thefirst angle is sufficient to generate spin torque electrons in the firstreference layer material at the WP-STT start time. The spin torqueelectrons generated in the first reference layer material is sufficientto initiate a process of switching the switchable magnetizationdirection at the WP-STT start time. The second fixed magnetizationdirection is configured to form a second angle between the second fixedmagnetization direction and the switchable magnetization direction. Thesecond angle is substantially insufficient to generate spin torqueelectrons in the second reference layer material at the WP-STT starttime. Advantages of the above-described embodiments of the inventioninclude, but are not limited to, improved WER performance. Morespecifically, because the above-described embodiments of the presentinvention provide spin torque electrons from the start of the writepulse, as well as a handoff process to complete the write operation, thelikelihood of a write error is less than in spin torque MTJconfigurations that do not provide spin torque electrons from the startof the write pulse. Accordingly, spin torque MTJ storage elementsaccording to the above-described embodiments of the invention provideimproved WER performance.

Embodiments of the invention are directed to a MTJ storage element. In anon-limiting example, the MTJ includes a first reference layer formedfrom a first reference layer material having a first fixed magnetizationdirection and a predetermined Curie temperature, along with a free layerformed from a free layer material having a switchable magnetizationdirection. The MTJ further includes a second reference layer formed froma second reference layer material having a second fixed magnetizationdirection. The MTJ storage element is configured to receive a WP-STTstart time, a WP-STT start segment duration and a write pulse duration.The WP-STT start segment duration is less than the write pulse duration.The MTJ storage element and the write pulse are configured to initiate aprocess of imparting Joule heating to the MTJ storage element when thewrite pulse is applied to the MTJ storage element at the WP-STT starttime. The Joule heating does not raise the temperature of the firstreference layer above the predetermined Curie temperature during theWP-STT start segment duration. The first fixed magnetization directionis configured to form a first angle between the first fixedmagnetization direction and the switchable magnetization direction. Thefirst angle is sufficient to generate spin torque electrons in the firstreference layer material at the WP-STT start time. The spin torqueelectrons generated in the first reference layer material is sufficientto initiate a process of switching the switchable magnetizationdirection at the WP-STT start time. The second fixed magnetizationdirection is configured to form a second angle between the second fixedmagnetization direction and the switchable magnetization direction. Thesecond angle is substantially insufficient to generate spin torqueelectrons in the second reference layer material at the WP-STT starttime. Advantages of the above-described embodiments of the inventioninclude, but are not limited to, improved WER performance. Morespecifically, because the above-described embodiments of the presentinvention provide spin torque electrons from the start of the writepulse, as well as a temperature-based handoff process to complete thewrite operation, the likelihood of a write error is less than in spintorque MTJ configurations that do not provide spin torque electrons fromthe start of the write pulse. Accordingly, spin torque MTJ storageelements according to the above-described embodiments of the inventionprovide improved WER performance.

Embodiments of the invention are directed to a method of operating a MTJstorage element. In a non-limiting example of the method, the MTJstorage element includes a reference layer and a free layer. Thereference layer includes a reference layer material having a fixedmagnetization direction, and the free layer includes a free layermaterial having a switchable magnetization direction. The methodincludes receiving a write pulse at the MTJ storage element to generatespin torque electrons. The write pulse includes a WP-STT start time, aWP-STT start segment duration and a write pulse duration. The WP-STTstart segment duration is less than the write pulse duration. The fixedmagnetization direction is configured to form an angle between the fixedmagnetization direction and the switchable magnetization direction. Theangle is sufficient to generate, based at least in part on receiving thewrite pulse, the spin torque electrons in the reference layer materialat the WP-STT start time. The method further includes initiating, basedat least in part on the spin torque electrons generated in the referencelayer material, a process of switching the switchable magnetizationdirection at the WP-STT start time. Advantages of the above-describedembodiments of the invention include, but are not limited to, improvedWER performance. More specifically, because the above-describedembodiments of the present invention provide spin torque electrons fromthe start of the write pulse, the likelihood of a write error is lessthan in spin torque MTJ configurations that do not provide spin torqueelectrons from the start of the write pulse. Accordingly, spin torqueMTJ storage elements according to the above-described embodiments of theinvention provide improved WER performance.

Embodiments of the invention are directed to a method of forming a MTJstorage element. In a non-limiting example, the method includes forminga first reference layer, a second reference layer, and a free layerbetween the first reference layer and the second reference layer. Themethod further includes forming the first reference layer from a firstreference layer material having a first fixed magnetization direction.The method further includes forming the free layer from a free layermaterial having a switchable magnetization direction. The method furtherincludes forming the second reference layer from a second referencelayer material having a second fixed magnetization direction. The methodfurther includes configuring the MTJ to receive a WP-STT start time, aWP-STT start segment duration and a write pulse duration. The WP-STTstart segment duration is less than the write pulse duration. The methodfurther includes configuring the first fixed magnetization direction toform a first angle between the first fixed magnetization direction andthe switchable magnetization direction. The first angle is sufficient togenerate spin torque electrons in the first reference layer material atthe WP-STT start time. The spin torque electrons generated in the firstreference layer material is sufficient to initiate a process ofswitching the switchable magnetization direction at the WP-STT starttime. The method further includes configuring the second fixedmagnetization direction to form a second angle between the second fixedmagnetization direction and the switchable magnetization direction. Thesecond angle is substantially insufficient to generate spin torqueelectrons in the second reference layer material at the WP-STT starttime. Advantages of the above-described embodiments of the inventioninclude, but are not limited to, improved WER performance. Morespecifically, because the above-described embodiments of the presentinvention provide spin torque electrons from the start of the writepulse, the likelihood of a write error is less than in spin torque MTJconfigurations that do not provide spin torque electrons from the startof the write pulse. Accordingly, spin torque MTJ storage elementsaccording to the above-described embodiments of the invention provideimproved WER performance.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedsubject matter. For a better understanding, refer to the description andto the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The diagrams depicted herein are illustrative. There can be manyvariations to the diagram or the operations described therein withoutdeparting from the spirit of the invention. For instance, the actionscan be performed in a differing order or actions can be added, deletedor modified. Also, the term “coupled” describes having a signal pathbetween two elements and does not imply a direct connection between theelements with no intervening elements/connections therebetween. All ofthese variations are considered a part of the specification.

The subject matter of the invention is particularly pointed out anddistinctly claimed in the claims at the conclusion of the specification.The foregoing and other features and advantages are apparent from thefollowing detailed description taken in conjunction with theaccompanying drawings in which:

FIG. 1 depicts a block diagram of a STT-MRAM capable of utilizing athermally-assisted spin torque MTJ storage element configured accordingto embodiments of the present invention;

FIG. 2 depicts a block diagram of a thermally-assisted spin torque MTJconfigured according to embodiments of the present invention;

FIG. 3A depicts a various segments of write pulse that can be applied toa thermally-assisted spin torque MTJ configured according to embodimentsof the invention;

FIG. 3B depicts a sequence of diagrams illustrating a write operation ofa thermally-assisted spin torque MTJ configured according to embodimentsof the invention; and

FIG. 4 depicts a flow diagram illustrating a method of forming athermally-assisted spin torque MTJ according to embodiments of theinvention.

DETAILED DESCRIPTION

Various embodiments of the present invention are described herein withreference to the related drawings. Alternative embodiments of theinvention can be devised without departing from the scope of thisinvention. It is noted that various connections and positionalrelationships (e.g., over, below, adjacent, etc.) are set forth betweenelements in the following description and in the drawings. Theseconnections and/or positional relationships, unless specified otherwise,can be direct or indirect, and the present invention is not intended tobe limiting in this respect. Accordingly, a coupling of entities canrefer to either a direct or an indirect coupling, and a positionalrelationship between entities can be a direct or indirect positionalrelationship. As an example of an indirect positional relationship,references in the present description to forming layer “A” over layer“B” include situations in which one or more intermediate layers (e.g.,layer “C”) is between layer “A” and layer “B” as long as the relevantcharacteristics and functionalities of layer “A” and layer “B” are notsubstantially changed by the intermediate layer(s).

For the sake of brevity, conventional techniques related to MTJfabrication may or may not be described in detail herein. Moreover, thevarious tasks and process steps described herein can be incorporatedinto a more comprehensive procedure or process having additional stepsor functionality not described in detail herein. In particular, varioussteps in the manufacture of STT-MRAM and MTJ devices are well known andso, in the interest of brevity, many conventional steps are onlymentioned briefly herein or are omitted entirely without providing thewell-known process details.

As previously described herein, in STT-MRAM, the spin torque that isapplied to the free layer is proportional to the sine of θ, where θ isthe angle between the free layer magnetization direction and thereference layer magnetization direction. In a zero temperatureenvironment, when the STT-MRAM switching process starts, there is nospin torque because θ is equal to zero (0) degrees (i.e., themagnetization directions of the fixed and free layers are parallel) or180 degrees (i.e., the magnetization directions of the fixed and freelayers are anti-parallel), and the sine of zero (0) degrees or 180degrees is zero (0). In practice, however, thermal fluctuationstypically move the free layer magnetization by very small amounts. Thethermal fluctuations oscillate the free layer back and forth by a verysmall amount. Thus, in practice, the sine θ term at the start of thewrite process is not precisely zero (0) so spin torque switching canoccur as long as a thermal fluctuation comes along when the switchingcurrent is applied. However, occasionally, no thermal fluctuations comealong and in that case sin θ is zero (0), and so the bit does notswitch. This spin torque incubation period, which is inherent to knownSTT-MRAM configurations, results in inherent switching delays that canincrease the WER. Accordingly, a contributing factor to WER inthermally-assisted MTJ storage elements is that no or insufficient spintorque is generated at the start of the write operation because sine θ,at the start of a write operation and for a period of time thereafter,is either zero (0) or very close to zero (0).

Turning now to an overview of aspects of the invention, embodiments ofthe invention address the above-described WER shortcomings by providingnovel configurations, fabrication methods, and operation methods for theMTJ storage element of a STT-MRAM, which, in some embodiments, is athermally assisted STT-MRAM. In an example embodiment of the invention,a bottom tunnel barrier is provided between a bottom reference/fixedlayer and a free layer. A top tunnel barrier is provided over the freelayer, and a top reference/fixed layer is provided over the top tunnelbarrier. In embodiments of the invention, the symbol θ identifies theangle between the free layer magnetization direction and the topreference/fixed layer magnetization direction, and the symbol θ′identifies the angle between the free layer magnetization direction andthe bottom reference/fixed layer magnetization direction.

According to aspects of the invention, the top reference/fixed layer'smagnetization direction is set such that the angle θ is at neither zero(0) degrees nor one hundred and eighty (180) degrees. In someembodiments of the invention, the top reference/fixed layer'smagnetization direction is set such that the angle θ is sufficientlylarge to produce spin torque electrons in the top reference/fixed layerat the start of a write operation, thus improving WER performance. Insome embodiments of the invention, the top reference/fixed layer'smagnetization direction is substantially in-plane with the respect tothe top reference/fixed layer and substantially perpendicular withrespect to the free layer's magnetization direction. Because the topreference/fixed layer's magnetization direction is substantiallyperpendicular with respect to the free layer's magnetization direction,the angle θ is an example of an angle θ that is sufficiently large toproduce spin torque electrons in the top reference/fixed layer at thestart of a write operation, thus improving WER performance. In someembodiments of the invention, the spin torque electrons generated in thetop reference/fixed layer begin the process of switching the freelayer's magnetization direction by tunneling through a top tunnelbarrier into the free layer.

As the free layer's magnetization direction continues to switch underthe influence of spin torque electrons from the top reference/fixedlayer, the angle θ′, which is the angle between the free layermagnetization direction and the bottom reference/fixed layermagnetization direction, increases. At a handoff time (e.g., 1 ns) afterthe start of the write operation, the angle θ′ is sufficiently large toproduce enough spin torque electrons in the bottom reference/fixed layerthat the bottom reference/fixed layer is now switching the free layer'smagnetization direction. During the portion of the write pulse after thehandoff time, the bottom reference/fixed layer completes the process ofswitching the free layer's magnetization direction. Because embodimentsof the present invention provide spin torque electrons from the start ofthe write pulse, the likelihood of a write error is less than in spintorque MTJ configurations that do not provide spin torque electrons fromthe start of the write pulse. Accordingly, spin torque MTJ storageelements according to embodiments of the invention provide improved WERperformance.

It is contemplated that handing the primary influence on the freelayer's magnetization direction off to the bottom reference/fixed layercan be implemented in a variety of ways that would fall within the scopeof the present invention. In some embodiments of the invention, the handoff can be executed in the following manner. The top reference/fixedlayer can be provided with a low Tc (i.e., Curie temperature) of about150 Celsius degrees. As previously noted herein, a material's Curietemperature is the temperature at which the material becomes nonmagneticor de-magnetized. When the write pulse is first applied, the Jouleheating from the applied current has not generated enough heat to raisethe MTJ the storage element's temperature above 150 Celsius. Therefore,up to the handoff time (e.g., the first one (1) ns of a ten (10) nswrite pulse), the top reference/fixed layer remains magnetic and appliesspin torque to the free layer to efficiently move the free layer'smagnetization direction to be parallel with the top reference/fixedlayer's magnetization direction. At the handoff time (e.g., about one(1) ns), the Joule heating from the applied current heats the MTJ'sstorage element above 150 Celsius, which makes the top reference/fixedlayer nonmagnetic. When the top reference/fixed layer is nonmagnetic,the generation of spin torque electrons in the top reference/fixed layerceases.

The angle θ′ is at or near zero at the start of the write pulse.Accordingly, at the start of the write pulse, the angle θ′ starts at ornear zero. As the top reference layer continues to the process ofswitching the free layer magnetization direction, the angle θ′ increasesover time. By about the handoff time (e.g., about one (1) ns), the angleθ′ has become sufficiently large to produce enough spin torque electronsin the bottom reference/fixed layer that the bottom reference/fixedlayer can take over switching the free layer's magnetization directionwhen the top reference/fixed layer becomes nonmagnetic and the spintorque from the top reference/fixed layer ceases. During the portion ofthe write pulse after the handoff time, the bottom reference/fixed layercompletes the process of switching the free layer's magnetizationdirection. Accordingly, in contrast to known STT-MRAM, methods andstructures of the invention generate sufficient spin torque electrons tobegin switching the free layer magnetization direction from the verybeginning of the write pulse, thus improving WER performance.

In some embodiments of the invention, the above-describedcharacteristics and roles of the top reference layer and the bottomreference layer can be reversed.

Turning now to a more detailed description of aspects of the presentinvention, FIG. 2 depicts a block diagram illustrating an exampleconfiguration of a thermally-assisted spin torque MTJ-based storageelement 102A according to embodiments of the present invention. The MTJ102A can be implemented in the STT-MRAM 100 (shown in FIG. 1) in thesame manner as the MTJ 102. The MTJ 102A includes a bottom magneticreference layer 204 (e.g., Fe, CoFe, CoFeB, etc.), a bottom dielectrictunnel barrier 206 (e.g., MgO), a free magnetic layer 208 (e.g., Fe,CoFe, CoFeB, etc.), a top dielectric tunnel barrier 210 (e.g., MgO), anda top magnetic reference layer 212 (e.g., Fe, CoFe, CoFeB, etc.),configured and arranged as shown. In some embodiments of the invention,the positions of the top reference layer 212 and the bottom referencelayer 204 shown in FIG. 1 can be reversed.

The bottom magnetic reference layer 204 and the top magnetic referencelayer 212 are formed and configured such that their respectivemagnetization directions 220, 224 are fixed. The free magnetic layer 208is formed and configured in a manner that provides it with a switchablemagnetization direction 222.

The top tunnel barrier layer 210 can be formed from a relatively thin(e.g., about 10 angstroms) layer of dielectric material (e.g., MgO).When two conducting electrodes (e.g., top reference/fixed layer 212 andfree layer 208) are separated by a thin dielectric layer (e.g., toptunnel barrier layer 210), electrons can tunnel through the dielectriclayer resulting in electrical conduction. The electron tunnelingphenomenon arises from the wave nature of the electrons, and theresulting junction electrical conductance is determined by theevanescent state of the electron wave function within the tunnelbarrier. Accordingly, the top tunnel barrier 210 is configured to bethin enough to allow electrons (specifically, spin torque electrons)from the top reference layer 212 to quantum mechanically tunnel throughthe top tunnel barrier 210. The top tunnel barrier 210 is alsoconfigured to be thick enough to decouple the free layer 208 from thetop reference layer 212 such that the magnetization direction 222 of thefree layer 208 is free to flip back and forth.

Similarly, the bottom tunnel barrier layer 206 can be formed from arelatively thin (e.g., about 10 angstroms) layer of dielectric material(e.g., MgO). The bottom tunnel barrier 206 is configured to be thinenough to allow electrons (specifically, spin torque electrons) from thebottom reference layer 204 to quantum mechanically tunnel through thebottom tunnel barrier 206. The bottom tunnel barrier 206 is alsoconfigured to be thick enough to decouple the free layer 208 from thebottom reference layer 204 such that the magnetization direction 222 ofthe free layer 208 is free to flip back and forth.

In the embodiment of the invention depicted in FIG. 2, the magnetizationdirection 224 of the top reference layer 212 is formed and configured tobe substantially in-plane with the respect to the top reference layer212 and substantially perpendicular with respect to the magnetizationdirection 222 of the free layer 208. Because the magnetization direction224 of the top reference layer 212 is substantially perpendicular withrespect to the magnetization direction 222 of the free layer 208, thetop reference layer 212 is provided with an angle θ, where θ is theangle between the free layer magnetization direction 222 and the topreference/fixed layer magnetization direction 224. A diagram showing asnapshot of the angle θ for the MTJ 102A at the start of the writeoperation is shown in FIG. 2. The angle θ is sufficiently large togenerate enough spin torque electrons in the top reference layer 212 totunnel through the top tunnel barrier 212 into the free layer 208 at thestart of a write operation. In embodiments of the invention, the spintorque electrons generated in the top reference/fixed layer 212 by theangle θ are sufficient to begin switching the magnetization direction222 of the free layer 208 at the very start of the write operation(i.e., at the very beginning of the write pulse), thus improving the WERperformance. In some embodiments of the invention, the angle θ isselected to be neither zero (0) degrees nor one hundred and eighty (180)degrees. In some embodiments of the invention, the angle θ is selectedbe large enough to generate enough spin torque electrons generated inthe top reference/fixed layer 212 to begin switching the magnetizationdirection 222 of the free layer 208 at the very start of the writeoperation (i.e., at the very beginning of the write pulse), thusimproving the WER performance.

The previously described angle θ′, which is the angle between the freelayer magnetization direction 222 and the bottom reference/fixed layermagnetization direction 220, is at or near zero at the start of thewrite pulse. A diagram showing a snapshot of the angle θ′ for the MTJ102A at the start of the write operation is shown in FIG. 2. As the freelayer's magnetization direction (M2) 222 continues to switch under theinfluence of spin torque electrons from the top reference/fixed layer212, the angle θ′ changes over time. At a handoff time (e.g., 1 ns)after the start of the write operation, the angle θ′ has changedsufficiently to produce enough spin torque electrons in the bottomreference/fixed layer 204 that the bottom reference/fixed layer 204 isnow switching the free layer's magnetization direction 222. During theportion of the write pulse after the handoff time, the bottomreference/fixed layer 204 completes the process of switching the freelayer's magnetization direction 222. Because embodiments of the presentinvention provide spin torque electrons from the start of the writepulse, the likelihood of a write error is less than in spin torque MTJconfigurations that do not provide spin torque electrons from the startof the write pulse. Accordingly, spin torque MTJ storage elementsconfigured and operated according to embodiments of the invention (e.g.,MTJ 102A) provide improved WER performance.

It is contemplated that handing the primary influence on the freelayer's magnetization direction 222 off to the bottom reference/fixedlayer 204 can be implemented in a variety of ways that would fall withinthe scope of the present invention. In some embodiments of theinvention, the hand off can be executed in the following manner. The topreference/fixed layer 212 can be provided with a low Tc (i.e., Curietemperature) of about 150 Celsius degrees. As previously noted herein, amaterial's Curie temperature is the temperature at which the materialbecomes nonmagnetic or de-magnetized. When the write pulse is firstapplied, the Joule heating from the applied current has not generatedenough heat to raise the temperature of the MTJ 102A above 150 Celsius.Therefore, up to the handoff time (e.g., the first one (1) ns of a ten(10) ns write pulse), the top reference/fixed layer 212 remains magneticand applies spin torque electrons to the free layer 208 to efficientlymove the free layer's magnetization direction 222 to be parallel withthe top reference/fixed layer's magnetization direction 224. At thehandoff time (e.g., about one (1) ns), the Joule heating from theapplied current heats the MTJ 102A above 150 Celsius, which makes thetop reference/fixed layer 212 nonmagnetic. When the top reference/fixedlayer 212 is nonmagnetic, the generation of spin torque electrons in thetop reference/fixed layer 212 ceases.

A write operation of the MTJ 102A will now be described with referenceto FIGS. 3A and 3B. FIG. 3A depicts a diagram of a write pulse 350according to embodiments of the invention. The write pulse 350 (shown inFIG. 3A) is applied to the MTJ 104A (shown in FIGS. 2 and 3B) andoperates, according to embodiments of the invention, to change themagnetization direction 222 of the free layer 208 from pointing down topointing up (or from pointing up to pointing down), thereby writing tothe MTJ 104A. FIG. 3B depicts a sequence of diagrams illustrating howthe magnetization directions 220, 222, 224 of the MTJ 102A change overtime during the initial application of a write pulse (i.e., switchingcurrent) according to embodiments of the invention.

Turning initially to FIG. 3A, the write pulse 350 is depicted in adiagram/graph that shows how the magnitude (M) of the write pulse 350changes over time (t). In the depicted embodiment, the write pulse 350includes a write pulse duration 352 of about ten (10) nanoseconds, whichitself includes a start segment duration 354 (about one (1) nanosecond)and a handoff segment duration 356 (about nine (9) nanoseconds). Thewrite pulse 350 and the start segment duration 354 start at t=0, whichis designated as the write-pulse and spin-transfer-torque (WP-STT) starttime. The end of the start segment duration 354 is at the handoff time,which is shown in FIG. 3B at t=one (1) nanosecond. The handoff timemarks the beginning of the handoff segment duration 356. The end of thehandoff segment 356 and the end of the write pulse duration 352 are atthe write pulse end time, which is shown in FIG. 3A at t=ten (10)nanoseconds.

FIG. 3B depicts a sequence of four diagrams along the top of FIG. 3B,where each diagram illustrates the magnetization directions 220, 222,224 of the MTJ 102A at a particular time (t) and with an upward currentdirection (e-) during application of the write pulse 350 (shown in FIG.3A). More specifically, the magnetization directions 220, 222, 224 ofthe MTJ 102A are depicted at t=zero (0) ns, t=0.9 ns, t=1.1 ns, and t=10ns. Thus, the diagram at t=zero (0) depicts the magnetization directions220, 222, 224 of the MTJ 102A at the WP-STT start time (shown in FIG.3A). The diagram at t=0.9 ns depicts the magnetization directions 220,222, 224 of the MTJ 102A just before the handoff time (t=one (1) ns,shown in FIG. 3A). The diagram at t=1.1 ns depicts the magnetizationdirections 220, 222, 224 of the MTJ 102A just after the handoff time(t=one (1) ns, shown in FIG. 3A). The diagram at t=10 ns depicts themagnetization directions 220, 222, 224 of the MTJ 102A at the writepulse end time (t=10 ns, shown in FIG. 3A).

Referring now to the sequence of diagrams shown along the top of FIG.3B, the write pulse 350 (shown in FIG. 3A) is applied to the MTJ 104Aand operates, according to embodiments of the invention, to change themagnetization direction 222 of the free layer 208 from pointing down topointing up, thereby writing to the MTJ 104A. The diagram at t=zero (0)ns depicts the magnetization directions 220, 222, 224 of the MTJ 102A atthe WP-STT start time (shown in FIG. 3A). At the WP-STT start time, themagnetization direction 222 of the free layer 208 is anti-parallel withrespect to the magnetization direction 220 of the bottom reference layer204. Accordingly, the angle θ between the magnetization direction 222and the magnetization direction 220 is substantially zero (0), and thebottom reference layer 204 initially produces either no or aninsufficient amount of spin torque electrons to switch the polarity ofthe free layer magnetization direction 222. In contrast, at the WP-STTstart time, the magnetization direction 222 of the free layer 208 issubstantially perpendicular with respect to the substantially in-planemagnetization direction 224 of the top reference layer 224. Accordingly,the angle θ between the magnetization direction 222 and themagnetization direction 224 is about 90 degrees, and the top referencelayer 212 initially produces a sufficient amount of spin torqueelectrons to initiate a process of switching the polarity of the freelayer magnetization direction 222.

As the above-described process of switching the direction of the freelayer magnetization direction 222 continues, the angle θ′ between themagnetization direction 222 and the magnetization direction 220 beginsto change over time. Accordingly, although at the WP-STT start time thebottom reference layer 204 produces either no or insufficient spintorque electrons to switch the magnetization direction 222, at somepoint into the above-described process of switching the polarity of thefree layer magnetization direction 222, the sine θ′ value changes enoughthe bottom reference layer 204 begins to produce sufficient spin torqueelectrons to assist the above described process of switching thepolarity of the free layer magnetization direction 222.

The diagram at t=0.9 ns depicts the magnetization directions 220, 222,224 of the MTJ 102A just before the handoff time (t=one (1) ns, shown inFIG. 3A). At t=0.9 ns, the magnetization direction 222, under theinfluence of spin torque supplied by the top reference layer 212 and thebottom reference layer 204, has been switched to the position where itis substantially anti-parallel with respect to the magnetizationdirection 224 of the top reference layer 212, as well as substantiallyperpendicular with respect to the fixed magnetization direction 220.Thus, at t=0.9 ns, the top reference layer 212 is attempting to maintainan anti-parallel relationship between the magnetization direction 222and the magnetization direction 224, and, at the same time, the bottomreference layer 204 is attempting to switch the magnetization direction222 into a parallel relationship between the magnetization direction 222and the magnetization direction 220.

Passing from t=0.9 ns to t=1.1 ns, the MTJ 104A passes through thehandoff time, which is t=one (1) ns as shown in FIG. 3A. At the handofftime, the influence of the top reference layer 212 on the free layermagnetization direction 222 is removed, and the primary influence on thedirection of the free layer magnetization direction 222 is handed off tothe fixed magnetization direction 220 of the bottom reference layer 204.

In a non-limiting embodiment of the invention, the influence of the topreference layer 212 on the free layer magnetization direction 222 can beremoved according to the following example methodology. The topreference layer 212 has been configured with a low Tc (i.e., Curietemperature) of, for example, about 150 Celsius degrees. As previouslynoted herein, a material's Curie temperature is the temperature at whichthe material becomes nonmagnetic. At about t=one (1) ns, the Jouleheating from the write pulse 350 heats the MTJ 104A to above 150Celsius, which makes the top reference layer 212 nonmagnetic. Hence, asdepicted, at t=1.1 ns, the top reference layer 212 has been converted tononmagnetic material, and the magnetization direction 224 of the topreference layer 212 has been removed.

At t=1.1 ns, the angle θ between the magnetization direction 222 and themagnetization direction 220 is about 90 degrees, and the bottomreference layer 204 now produces a sufficient amount of spin torqueelectrons to take over and complete the process of switching thepolarity of the free layer magnetization direction 222. By t=10 ns,which is the end of the write pulse 350 (shown in FIG. 3A), the processof switching the polarity of the free layer magnetization direction 222has been completed, the free layer magnetization direction 222 issubstantially parallel with respect to the reference layer magnetizationdirection 220, and the write process is complete.

Using the switching methodologies of the present invention, the inherentWER of known STT-MRAM configurations is overcome by providing, at thevery start of the write pulse 350, an angle θ between the free layermagnetization direction 222 and the magnetization direction 224 thatproduces from the very start of the write operation a sufficient amountof spin torque electrons to initiate a process of switching the polarityof the free layer magnetization direction 222. Because a primary causeof WER in known thermal STT-MRAM designs is that sine θ is either zero(0) or very close to zero (0) at the start of the spin torque switching,the switching methodologies of the present invention address and improve(i.e., decrease) the inherent WER of known STT-MRAM configurations.

FIG. 3B also depicts a sequence of four diagrams along the bottom ofFIG. 3B, where each diagram illustrates the magnetization directions220, 222, 224 of the MTJ 102A at a particular time (t) and with adownward current direction (e-) during application of a negative versionof the write pulse 350 (shown in FIG. 3A). The switching operationdepicted by the sequence of four diagrams along the bottom of FIG. 3Bproceeds in substantially the same manner as the switching operationdepicted by the sequence of four diagrams along the top of FIG. 3B,except the current direction is downward, and the free layermagnetization direction 222 is switched from pointing upward to pointingdownward.

Referring now to the sequence of diagrams shown along the bottom of FIG.3B, a negative version of the write pulse 350 (shown in FIG. 3A) isapplied to the MTJ 104A and operates, according to embodiments of theinvention, to change the magnetization direction 222 of the free layer208 from pointing up to pointing down, thereby writing to the MTJ 104A.The diagram at t=zero (0) ns depicts the magnetization directions 220,222, 224 of the MTJ 102A at the WP-STT start time (shown in FIG. 3A). Atthe WP-STT start time, the magnetization direction 222 of the free layer208 is anti-parallel with respect to the magnetization direction 220 ofthe bottom reference layer 204. Accordingly, the angle θ between themagnetization direction 222 and the magnetization direction 220 issubstantially zero (0), and the bottom reference layer 204 initiallyproduces either no or an insufficient amount of spin torque electrons toswitch the polarity of the free layer magnetization direction 222. Incontrast, at the WP-STT start time, the magnetization direction 222 ofthe free layer 208 is substantially perpendicular with respect to thesubstantially in-plane magnetization direction 224 of the top referencelayer 224. Accordingly, the angle θ between the magnetization direction222 and the magnetization direction 224 is about 90 degrees, and the topreference layer 212 initially produces a sufficient amount of spintorque electrons to initiate a process of switching the polarity of thefree layer magnetization direction 222.

As the above-described process of switching the polarity of the freelayer magnetization direction 222 continues, an angle θ begins todevelop and continuously increases between the magnetization direction222 and the magnetization direction 220. Accordingly, although at theWP-STT start time the bottom reference layer 204 produces either no orinsufficient spin torque electrons to switch the magnetization direction222, at some point into the above-described process of switching thepolarity of the free layer magnetization direction 222, the bottomreference layer 204 begins to produce sufficient spin torque electronsto assist the above described process of switching the polarity of thefree layer magnetization direction 222.

The diagram at t=0.9 ns depicts the magnetization directions 220, 222,224 of the MTJ 102A just before the handoff time (t=one (1) ns, shown inFIG. 3A). At t=0.9 ns, the magnetization direction 222, under theinfluence of spin torque supplied by the top reference layer 212 and thebottom reference layer 204, has been switched to the position where itis substantially parallel with respect to the magnetization direction224 of the top reference layer 212, as well as substantiallyperpendicular with respect to the fixed magnetization direction 220.Thus, at t=0.9 ns, the top reference layer 212 is attempting to maintaina parallel relationship between the magnetization direction 222 and themagnetization direction 224, and, at the same time, the bottom referencelayer 204 is attempting to switch the magnetization direction 222 intoan anti-parallel relationship between the magnetization direction 222and the magnetization direction 220.

Passing from t=0.9 ns to t=1.1 ns, the MTJ 104A passes through thehandoff time, which is t=one (1) ns (shown in FIG. 3A). At the handofftime, the influence of the top reference layer 212 on the free layermagnetization direction 222 is removed, and the primary influence on thedirection of the free layer magnetization direction 222 is handed off tothe fixed magnetization direction 220 of the bottom reference layer 204.

In a non-limiting embodiment of the invention, the influence of the topreference layer 212 on the free layer magnetization direction 222 can beremoved according to the following example methodology. The topreference layer 212 has been configured with a low Tc (i.e., Curietemperature) of, for example, about 150 Celsius degrees. As previouslynoted herein, a material's Curie temperature is the temperature at whichthe material becomes nonmagnetic. At about t=one (1) ns, the Jouleheating from the negative version of the write pulse 350 heats the MTJ104A to above 150 Celsius, which makes the top reference layer 212nonmagnetic. Hence, as depicted, at t=1.1 ns, the top reference layer212 has been converted to nonmagnetic material, and the magnetizationdirection 224 of the top reference layer 212 has been removed.

At t=1.1 ns, the angle θ between the magnetization direction 222 and themagnetization direction 220 is about 90 degrees, and the bottomreference layer 204 now produces a sufficient amount of spin torqueelectrons to take over and complete the process of switching thepolarity of the free layer magnetization direction 222. By t=10 ns,which is the end of the negative version of the write pulse 350, theprocess of switching the polarity of the free layer magnetizationdirection 222 has been completed, the free layer magnetization direction222 is substantially anti-parallel with respect to the reference layermagnetization direction 220, and the write process is complete.

As previously noted herein, using the switching methodologies of thepresent invention, the inherent WER of known STT-MRAM configurations isovercome by providing, at the very start of the negative version of thewrite pulse 350, an angle θ between the free layer magnetizationdirection 222 and the magnetization direction 224 that produces from thevery start of the write operation a sufficient amount of spin torqueelectrons to initiate a process of switching the polarity of the freelayer magnetization direction 222. Because a primary cause of WER inknown thermal STT-MRAM designs is that sine θ is either zero (0) or veryclose to zero (0) at the start of the spin torque switching, theswitching methodologies of the present invention address and improve(i.e., decrease) the inherent WER of known STT-MRAM configurations.

Examples of suitable materials and configurations for the bottomreference layer 204 include, but are not limited to multilayers of Coand Ni, Co and Pt, Co and Pd, L10 alloys such as FePd, FePt, CoPd, andCoPt. In some embodiments of the invention, the reference layer (e.g.,204) also has a thin interface layer of CoFeB at the tunnel barrier(e.g., 206) MgO interface, separated from the rest of the referencelayer (for example Co/Pt multilayer) by an ultrathin Ta or otherrefractory metal layer. The reference layer can also include a syntheticantiferromagnet, which includes two magnetic layers separated by a thinRu spacer so that the two magnetic layers are aligned antiparallel.

Examples of suitable materials and configurations for the top referencelayer 212 include, but are not limited to an alloy containing at leastone of Fe, Co or Ni, for example CoFeB or CoFe.

Examples of suitable materials and configurations for the bottom tunnelbarrier layer 206 include, but are not limited to MgOx, AlOx, and TiOx.

Examples of suitable materials and configurations for the top tunnelbarrier layer 210 include, but are not limited to MgOx, AlOx, and TiOx.

The thicknesses of the respective layers of the MTJ 102A can varyaccording to design considerations. For example, the thicknesses of thelayers of the MTJ 102A can be designed to have predeterminedthicknesses, to have thicknesses within predetermined ranges, to havethicknesses having fixed ratios with respect to each other, or to havethicknesses based on any other consideration or combination ofconsiderations in accordance with the functionality described herein.For example, the top reference layer 212 can have a thickness in a rangefrom about 0.8 nm to about 10 nm. The bottom reference layer 204 canhave a thickness in a range from about 2 nm to about 10 nm. The toptunnel barrier layer 210 can have a thickness in a range from about 0.5nm to about 3 nm. The bottom tunnel barrier layer 206 can have athickness in a range from about 0.5 nm to about 3 nm.

In addition to the MTJ 102A depicted in FIG. 2, alternativeconfigurations include an MTJ where one or both of the reference layers204, 212 are formed from a synthetic antiferromagnet.

FIG. 4 depicts a flow diagram illustrating a method 400 of forming theMTJ 102A according to embodiments of the invention. In block 402, thebottom reference layer 204 is formed. The bottom reference layer 204 canbe formed, for example, by any suitable deposition, growth or otherformation process. The bottom reference layer 204 can be formed offerromagnetic material, including, but not limited to, multilayers of Coand Ni, Co and Pt, Co and Pd, L10 alloys such as FePd, FePt, CoPd, andCoPt. In some embodiments of the invention, the reference layer (e.g.,204) also has a thin interface layer of CoFeB at the tunnel barrier(e.g., 206) MgO interface, separated from the rest of the referencelayer (for example Co/Pt multilayer) by an ultrathin Ta or otherrefractory metal layer. The reference layer can also include a syntheticantiferromagnet, which includes two magnetic layers separated by a thinRu spacer so that the two magnetic layers are aligned antiparallel. Inaccordance with embodiments of the invention, the bottom reference layer204 is formed and configured in a manner that provides the fixedmagnetization direction 220.

In block 404, the bottom tunnel barrier layer 206 is formed. The bottomtunnel barrier layer 206 can be formed, for example, by any suitabledeposition, growth or other formation process, and can be anon-conductive material, including, but not limited to, MgOx, AlOx, andTiOx. In accordance with embodiments of the invention, the bottom tunnelbarrier 206 is configured to be thin enough to allow electrons(specifically, spin torque electrons) from the bottom reference layer204 to quantum mechanically tunnel through the bottom tunnel barrier206. The bottom tunnel barrier 206 is also configured to be thick enoughto decouple the free layer 208 from the bottom reference layer 204 suchthat the magnetization direction 222 of the free layer 208 is free toflip back and forth.

In block 406, the free layer 208 is formed. The free layer 208 can beformed, for example, by any suitable deposition, growth or otherformation process. The free layer 208 can be formed of ferromagneticmaterial, including, but not limited to, Fe, CoFe, and CoFeB. Inaccordance with embodiments of the invention, the free layer 208 isformed and configured in a manner that provides the switchablemagnetization direction 222.

In block 408, the top tunnel barrier layer 210 is formed. The top tunnelbarrier layer 210 can be formed, for example, by any suitabledeposition, growth or other formation process, and can be anon-conductive material, including, but not limited to, MgOx, AlOx, andTiOx. In accordance with embodiments of the invention, the top tunnelbarrier 210 is configured to be thin enough to allow electrons(specifically, spin torque electrons) from the top reference layer 212to quantum mechanically tunnel through the top tunnel barrier 210. Thetop tunnel barrier 210 is also configured to be thick enough to decouplethe free layer 208 from the top reference layer 212 such that themagnetization direction 222 of the free layer 208 is free to flip backand forth.

In block 410, the top reference layer 212 is formed. The top referencelayer 212 can be formed, for example, by any suitable deposition, growthor other formation process. The top reference layer 212 can be formed offerromagnetic material, including, but not limited to, Fe, CoFe, CoFeB.In some embodiments, the top reference layer 212 can be provided with alow Tc (i.e., Curie temperature) of about 150 Celsius degrees.

In accordance with embodiments of the invention, the top reference layer212 is formed and configured in a manner that provides the fixedmagnetization direction 224. In embodiments of the invention, themagnetization direction 224 is selected to provide an angle θ such thatthe value of sine θ is sufficiently large to produce spin torqueelectrons in the top reference layer 212 at the very start of the writeoperation. In embodiments of the invention, the magnetization direction224 is selected to provide an angle θ such that the value of sine θ issufficiently large to produce enough spin torque electrons in the topreference layer 212 to begin the process of switching the magnetizationdirection 222 of the free layer 208 at the very start of the writeoperation. In the embodiment shown in FIG. 2, the magnetizationdirection 224 is selected such that the angle θ is approximately 90degrees. However, other directions can be selected for magnetizationdirection 224 as long as the selected direction for magnetizationdirection 224 provides a sine θ value that is sufficiently large toproduce spin torque electrons in the top reference layer 212 at the verystart of the write operation.

In accordance with embodiments of the invention, the top reference layer212 can also be formed and configured in a manner that allows the stopreference layer 212 to hand off the primary influence on the freelayer's magnetization direction 222 to the bottom reference/fixed layer204. It is contemplated that handing the primary influence on the freelayer's magnetization direction 222 off to the bottom reference/fixedlayer 204 can be implemented in a variety of ways that would fall withinthe scope of the present invention. In some embodiments of theinvention, the hand off can be executed in the following manner. The topreference/fixed layer 212 can be provided with a low Tc (i.e., Curietemperature) of about 150 Celsius degrees. As previously noted herein, amaterial's Curie temperature is the temperature at which the materialbecomes nonmagnetic or de-magnetized. When the write pulse is firstapplied, the Joule heating from the applied current has not generatedenough heat to raise the temperature of the MTJ 102A above 150 Celsius.Therefore, up to the handoff time (e.g., the first one (1) ns of a ten(10) ns write pulse), the top reference/fixed layer 212 remains magneticand applies spin torque electrons to the free layer 208 to efficientlymove the free layer's magnetization direction 222 to be parallel withthe top reference/fixed layer's magnetization direction 224. At thehandoff time (e.g., about one (1) ns), the Joule heating from theapplied current heats the MTJ 102A above 150 Celsius, which makes thetop reference/fixed layer 212 nonmagnetic. When the top reference/fixedlayer 212 is nonmagnetic, the generation of spin torque electrons in thetop reference/fixed layer 212 ceases.

Thus it can be seen from the foregoing detailed description that thepresent invention provides magnetic tunnel junction (MTJ) memory cellshaving improved WER performance. Technical effects and benefits of theinvention include providing a MTJ with a first reference layer that isconfigured to provide spin torque electrons at the very start of thewrite pulse and extending through an initial segment of the write pulse.The spin torque electrons produced by the first reference layer aresufficient to begin the process of switching the magnetization directionof the MTJ free layer. The MTJ further includes a second reference layerthat is configured to take over the generation of spin torque electronsfrom the first reference layer at the end of the initial segment of thewrite pulse. The spin torque electrons produced by the second referencelayer are sufficient to complete the process of switching themagnetization direction of the MTJ free layer. Using the switchingmethodologies of the present invention, the inherent WER of knownSTT-MRAM MTJ configurations is overcome by providing, at the very startof the write pulse, the first reference layer that is configured toproduce from the very start of the write operation a sufficient amountof spin torque electrons to initiate a process of switching thedirection of the free layer magnetization direction. Because a primarycause of WER in known thermal STT-MRAM designs is that there is aninherent delay before spin torque electrons are produced, the switchingmethodologies of the present invention address and improve (i.e.,decrease) the inherent WER of known STT-MRAM configurations.

The terms “example” or “exemplary” are used herein to mean “serving asan example, instance or illustration.” Any embodiment or designdescribed herein as “exemplary” is not necessarily to be construed aspreferred or advantageous over other embodiments or designs. The terms“at least one” and “one or more” are understood to include any integernumber greater than or equal to one, i.e. one, two, three, four, etc.The terms “a plurality” are understood to include any integer numbergreater than or equal to two, i.e. two, three, four, five, etc. The term“connection” can include an indirect “connection” and a direct“connection.”

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedcan include a particular feature, structure, or characteristic, butevery embodiment may or may not include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

For purposes of the description hereinafter, the terms “upper,” “lower,”“right,” “left,” “vertical,” “horizontal,” “top,” “bottom,” andderivatives thereof shall relate to the described structures andmethods, as oriented in the drawing figures. The terms “overlying,”“atop,” “on top,” “positioned on” or “positioned atop” mean that a firstelement, such as a first structure, is present on a second element, suchas a second structure, where intervening elements such as an interfacestructure can be present between the first element and the secondelement. The phrase “direct contact” means that a first element, such asa first structure, and a second element, such as a second structure, areconnected without any intermediary conducting, insulating orsemiconductor layers at the interface of the two elements. It should benoted that the phrase “selective to,” such as, for example, “a firstelement selective to a second element,” means that a first element canbe etched and the second element can act as an etch stop. The terms“about,” “substantially,” “approximately,” and variations thereof, areintended to include the degree of error associated with measurement ofthe particular quantity based upon the equipment available at the timeof filing the application. For example, “about” can include a range of±8% or 5%, or 2% of a given value.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of onemore other features, integers, steps, operations, element components,and/or groups thereof. For example, a composition, a mixture, process,method, article, or apparatus that comprises a list of elements is notnecessarily limited to only those elements but can include otherelements not expressly listed or inherent to such composition, mixture,process, method, article, or apparatus.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form described. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated

While a preferred embodiment has been described, it will be understoodthat those skilled in the art, both now and in the future, can makevarious improvements and enhancements which fall within the scope of theclaims which follow.

1. A magnetic tunnel junction (MTJ) storage element comprising: areference layer comprising a reference layer material having a fixedmagnetization direction; and a free layer comprising a free layermaterial having a switchable magnetization direction; where the MTJ isconfigured to receive a write pulse having a write pulse duration; wherethe fixed magnetization direction is configured to form an angle betweenthe fixed magnetization direction and the switchable magnetizationdirection; where the angle is sufficient to generate spin torqueelectrons in the reference layer material during the write pulseduration; where the reference layer material is configured to convertfrom magnetic to nonmagnetic during the write pulse duration.
 2. The MTJstorage element of claim 1, where: the write pulse further includes awrite-pulse and spin-transfer-torque (WP-STT) start time and a WP-STTstart segment duration; the WP-STT start segment duration is less thanthe write pulse duration; the angle is sufficient to generate spintorque electrons in the reference layer material at the WP-STT starttime; and the spin torque electrons generated in the reference layermaterial are sufficient to initiate a process of switching theswitchable magnetization direction at the WP-STT start time.
 3. The MTJstorage element of claim 1, where: the write pulse further includes aSTT handoff time and a STT handoff segment duration; the STT handofftime is subsequent to the WP-STT start time; and the reference layermaterial is further configured to convert from magnetic to nonmagneticat about the STT handoff time.
 4. The MTJ storage element of claim 3,where the reference layer material is further configured to: no longerinclude the fixed magnetization direction after the reference layermaterial has converted from magnetic to nonmagnetic at about the STThandoff time; and no longer generate spin torque electrons after thereference layer material has converted from magnetic to nonmagnetic atabout the STT handoff time.
 5. The MTJ storage element of claim 4,where: subsequent to the STT handoff time, spin torque electrons aregenerated by at least one other region of the MTJ storage element; andthe at least one other region is distinct from the reference layermaterial.
 6. A magnetic tunnel junction (MTJ) storage elementcomprising: a first reference layer comprising a first reference layermaterial having a first fixed magnetization direction; a free layercomprising a free layer material having a switchable magnetizationdirection; and a second reference layer comprising a second referencelayer material having a second fixed magnetization direction; where theMTJ is configured to receive a write pulse having a write-pulse andspin-transfer-torque (WP-STT) start time, a WP-STT start segmentduration and a write pulse duration; where the WP-STT start segmentduration is less than the write pulse duration; where the first fixedmagnetization direction is configured to form a first angle between thefirst fixed magnetization direction and the switchable magnetizationdirection; where the first angle is sufficient to generate spin torqueelectrons in the first reference layer material during the write pulseduration; where the spin torque electrons generated in the firstreference layer material are sufficient to initiate a process ofswitching the switchable magnetization direction at the WP-STT starttime; where the second fixed magnetization direction is configured toform a second angle between the second fixed magnetization direction andthe switchable magnetization direction; where the second angle issubstantially insufficient to generate spin torque electrons in thesecond reference layer material at the WP-STT start time; where thefirst reference layer material is configured to convert from magnetic tononmagnetic during the write pulse duration.
 7. The MTJ storage elementof claim 6, where: the write pulse further includes a STT handoff timeand a STT handoff segment duration; the STT handoff time is subsequentto the WP-STT start time; and the first reference layer material isfurther configured to convert from magnetic to nonmagnetic at about theSTT handoff time.
 8. The MTJ storage element of claim 7, where the firstreference layer material is further configured to no longer include thefirst fixed magnetization direction after the first reference layermaterial has converted from magnetic to nonmagnetic at about the STThandoff time.
 9. The MTJ storage element of claim 8, where the firstreference layer material is further configured to no longer generatespin torque electrons after the first reference layer material hasconverted from magnetic to nonmagnetic at about the STT handoff time.10. The MTJ storage element of claim 9, where subsequent to the STThandoff time, spin torque electrons are generated by the secondreference layer material.
 11. The MTJ storage element of claim 9 wheresubsequent to the STT handoff time, and subsequent to the spin torqueelectrons generated in the first reference layer material havinginitiated the process of switching the switchable magnetizationdirection, the second angle is substantially sufficient to generate spintorque electrons in the second reference layer material.
 12. A magnetictunnel junction (MTJ) storage element comprising: a first referencelayer comprising a first reference layer material having a first fixedmagnetization direction and a predetermined Curie temperature; a freelayer comprising a free layer material having a switchable magnetizationdirection; and a second reference layer comprising a second referencelayer material having a second fixed magnetization direction; where theMTJ storage element is configured to receive a write pulse having awrite-pulse and spin-transfer-torque (WP-STT) start time, a WP-STT startsegment duration and a write pulse duration; where the WP-STT startsegment duration is less than the write pulse duration; where MTJstorage element and the write pulse are configured to initiate a processof imparting Joule heating to the MTJ storage element when the writepulse is applied to the MTJ storage element at the WP-STT start time;where the Joule heating does not raise a temperature of the firstreference layer above the predetermined Curie temperature during theWP-STT start segment duration; where the first fixed magnetizationdirection is configured to form a first angle between the first fixedmagnetization direction and the switchable magnetization direction;where the first angle is sufficient to generate spin torque electrons inthe first reference layer material at the WP-STT start time; where thespin torque electrons generated in the first reference layer materialare sufficient to initiate a process of switching the switchablemagnetization direction at the WP-STT start time; where the second fixedmagnetization direction is configured to form a second angle between thesecond fixed magnetization direction and the switchable magnetizationdirection; where the second angle is substantially insufficient togenerate, at the WP-STT start time, spin torque electrons in the secondreference layer material; where the first reference layer material isconfigured to convert from magnetic to nonmagnetic during the writepulse duration.
 13. The MTJ storage element of claim 12, where thesecond fixed magnetization direction is substantially in-plane withrespect to a plane of the second reference layer.
 14. The MTJ storageelement of claim 12, where the second fixed magnetization directionbecomes substantially perpendicular with respect to the switchablemagnetization direction during the write pulse duration.
 15. The MTJstorage element of claim 12, where: the write pulse further includes aSTT handoff time and a STT handoff segment duration; the STT handofftime is subsequent to the WP-STT start time; and the STT handoff timecomprises when the Joule heating has raised the temperature of the firstreference layer material above the predetermined Curie temperature toconvert the first reference layer material from magnetic to nonmagnetic.16. The MTJ storage element of claim 15, where the first reference layermaterial is further configured to no longer include the first fixedmagnetization direction after the first reference layer material hasconverted from magnetic to nonmagnetic at about the STT handoff time.17. The MTJ storage element of claim 16, where the first reference layermaterial is further configured to no longer generate spin torqueelectrons after the first reference layer material has converted frommagnetic to nonmagnetic at about the STT handoff time.
 18. The MTJstorage element of claim 17, where subsequent to the STT handoff time,spin torque electrons are generated by the second reference layermaterial.
 19. The MTJ storage element of claim 17 where subsequent tothe STT handoff time, and subsequent to the spin torque electronsgenerated in the first reference layer material having initiated theprocess of switching the switchable magnetization direction, the secondangle is substantially sufficient to generate spin torque electrons inthe second reference layer material.
 20. A method of operating amagnetic tunnel junction (MTJ) storage element, the method comprising:receiving a write pulse at the MTJ storage element to generate spintorque electrons; where the MTJ storage element comprises a referencelayer and a free layer; where the reference layer comprises a referencelayer material having a fixed magnetization direction; where the freelayer comprises a free layer material having a switchable magnetizationdirection; where the write pulse comprises a write pulse duration; wherethe fixed magnetization direction is configured to form an angle betweenthe fixed magnetization direction and the switchable magnetizationdirection; where the angle is sufficient to generate, based at least inpart on receiving the write pulse, the spin torque electrons in thereference layer material at the WP-STT start time; and initiating, basedat least in part on the spin torque electrons generated in the referencelayer material, a process of switching the switchable magnetizationdirection during the write pulse duration; where the reference layermaterial is configured to convert from magnetic to nonmagnetic duringthe write pulse duration.
 21. The method of claim 20, where: the writepulse further includes a STT handoff time and a STT handoff segmentduration; and the STT handoff time is subsequent to the WP-STT starttime; the method further comprising converting the reference layermaterial from magnetic to nonmagnetic at about the STT handoff time. 22.The method of claim 21, where, subsequent to the reference layermaterial having converted from magnetic to nonmagnetic at about the STThandoff time, the reference layer material no longer includes the fixedmagnetization direction.
 23. The method of claim 22, where the referencelayer material no longer generates spin torque electrons subsequent tothe reference layer material having converted from magnetic tononmagnetic at about the STT handoff time.
 24. The method of claim 23,where: subsequent to the STT handoff time, spin torque electrons aregenerated by at least one other region of the MTJ storage element; andthe at least one other region is distinct from the reference layermaterial.
 25. A magnetic tunnel junction (MTJ) storage elementcomprising: a first reference layer; a second reference layer; a freelayer between the first reference layer and the second reference layer;where the first reference layer comprises a first reference layermaterial having a first fixed magnetization direction; where the freelayer comprises a free layer material having a switchable magnetizationdirection; where the second reference layer comprises a second referencelayer material having a second fixed magnetization direction; where theMTJ storage element is configured to receive a write pulse having awrite pulse duration; where the first fixed magnetization direction isconfigured to form a first angle between the first fixed magnetizationdirection and the switchable magnetization direction; where the firstangle is sufficient to generate spin torque electrons in the firstreference layer material during a first portion of the write pulseduration; where the spin torque electrons generated in the firstreference layer material are sufficient to initiate a process ofswitching the switchable magnetization direction during the firstportion of the write pulse duration; where the second fixedmagnetization direction is configured to form a second angle between thesecond fixed magnetization direction and the switchable magnetizationdirection; where, based at least in part on the switchable magnetizationdirection passing a predetermined threshold, the second angle becomingsufficient to generate spin torque electrons in the second referencelayer material during a second portion of the write pulse duration.